Sometime in 2016 people from several states could have trouble getting on an airplane or into federal buildings because of a post-Sept. 11 law that tightened requirements for state-issued identification.

When that will happen and who will be affected won’t be completely clear until the Department of Homeland Security releases further details of how it will enforce the 2005 REAL ID Act — an announcement that could come in the next few days. In the meantime, an expert on the law says people in places such as Illinois and Missouri — where DHS this week essentially said time’s up for the states to comply — may want to get a passport.

Here’s a look at the act, what it requires and why some states will feel the impact while others won’t:

WHAT IS REAL ID?

Congress approved the REAL ID Act in 2005, following a recommendation from the commission formed to study the Sept. 11, 2001 terrorist attacks. The 9/11 commission said the country would be safer if there were minimum standards for government-issued identification such as driver’s licenses that are required to enter federal buildings or board commercial airplanes.

The act set those standards, which include requiring applicants to provide proof of identity and legal US residency and requiring states to use counterfeit-resistant security features in the IDs.

DHS originally gave states until 2009 to make necessary changes to their requirements and technology.

WHAT’S MY STATE’S STATUS?

At least 20 states and the District of Columbia have complied with the federal requirements, according to information posted on the DHS website.

In other states implementation has been delayed or derailed by concerns about cost, violations of privacy or overreaching by the federal government. Lawmakers in some states passed legislation opposing REAL ID; Minnesota and Missouri still have laws prohibiting them from complying.

At least nineteen states have until October 2016 to either comply or be granted another extension. Alaska, California, Illinois, Missouri, New Jersey, New Mexico, South Carolina and Washington have extensions only through Jan. 10.

This week, officials in Missouri and Illinois said they’d been notified that DHS will not grant them another extension beyond Jan. 10. Washington received a similar notice in October.

DHS’ online map provides the status of each state at www.dhs.gov/REAL-id-enforcement-brief, though as of Thursday it didn’t indicate which states had additional extensions denied. A DHS spokesperson didn’t respond to phone messages seeking comment Thursday.

WHAT IF MY STATE DOESN’T COMPLY?

DHS is enforcing the act in phases, starting with federal facilities such as nuclear power plants, laboratories and military bases.

That means that starting Jan. 10, federal facilities won’t accept driver’s licenses from Illinois, Missouri or any other state that isn’t approved for an extension beyond that date. (The facilities already don’t accept most licenses from Minnesota, which DHS already has deemed to be non-compliant).

The law doesn’t apply to federal courthouses, hospitals or health clinics, according to DHS, and people may be allowed entry with another valid form of ID such as a passport or military identification.

DHS has said it will extend the requirements to airports sometime in 2016, though the department hasn’t said when. It’s expected to make that announcement as early as next week, and DHS has said it will give a notice of at least 120 days before it takes effect.

That could give states such as Illinois and Missouri time to pass laws or take other steps toward implementation, possibly making DHS more amenable to granting another extension.

WILL I BE ABLE TO FLY?

That depends on many factors, such as how DHS rolls out the airport requirements and whether travelers have other valid forms of ID.

Andrew Meehan is policy director for Keeping IDentities Safe, a Washington, D.C.-based nonprofit that advocates for implementation of REAL ID.

He said that if history is any indication DHS will phase in the airport portion of the law, possibly starting with smaller airstrips in mid-2016 and expanding to larger airports that serve more travelers at a later date.

Meehan called it “laughable” that so many states haven’t gotten on board with the law after more than a decade. His advice to people in states like Illinois and Missouri is to get a passport.